pep8
Prevent side-effects on messages in tests:

Was causing a failure in Python 3+ where previous sending of the
message was causing the X-Actually-To and X-Actually-From headers to
be turned into email.header.Header objects.  Probably also should
support that, but the tests should definitely be using fresh messages.
fixed handling of long headers with multibyte unicode characters

closes #13 and Pylons/pyramid_mailer#22, also answers [Stack Overflow
Question](http://stackoverflow.com/questions/12959926/pyramid-mailer-subject-header-issue).